The present invention relates generally to a receiver used in wireless telecommunication systems, and more particularly, relates to a receiver that has a honeycomb baffle to reduce ambient light.
A typical optical wireless telecommunications system comprises an optical transmitter and an optical receiver, with associated electronics to modulate and demodulate data on a light beam. Often, receivers are basically telescopes.
An example of a configuration of a typical optical receiver is shown at 10 in FIG. 1. The optical receiver 10 comprises a cylindrical receiver tube 12 with a receiver aperture 15, (sometimes made of glass), at one end, and a system of internal mirrors 20 and 18 to collect and focus light to a small optical detection unit 22. In this optical receiver 10, a light beam 16 (the data communication signal) enters the receiver tube 12 at a receiver aperture 15. The mirrors 18 and 20 focus the light beam 16 onto a small area at detection unit 22. An example of such a traditional system is a Cassegrain telescope with detector electronics located at the focal plane.
The optical receivers used in wireless optical communication systems are often required to operate under varying background illumination conditions, i.e., at night or under direct sunlight. Most problems occur when the optical receiver is operating in direct sunlight, particularly where sunlight is entering the receiver aperture 15 at an angle close to that of the data communication signal 16. Several potential problems are caused by this situation. First, the photodetector may be saturated with incident sunlight. Second, the optical system will be heated resulting in thermal gradients and mechanical deformation. Third, the optical system may produce unwanted images of the sun that lead to unsafe external heating or a blinding effect.
The first two problems only affect performance or internal damage of the optical receiver 10. The third problem is a more serious concern since it relates to safety at the deployment site of the optical receiver. One situation where this problem is known to exist is in a conventional Cassegrain 2-mirror telescope, as described above in relation to FIG. 1. In this case, when ambient light enters the receiver aperture 15 slightly off-axis, so that some light reflected from the primary mirror 20 does not subsequently strike the secondary mirror 18, there is an image produced at the focal plane of the primary mirror. Depending on the field angle of the incident light and the particular optical design of the telescope system, non-vignetted transmission to this image can be substantial. In the case of the off-axis illumination being sunlight, an absorbing surface placed at the prime-focus image can become dangerously hot.
Conventional approaches to mitigate the problem described above include: (1) incorporating a baffle aperture that limits the field of view of the optical receiver 10, (2) placing a band-pass filter in front of the receiver aperture 15 to reject wavelengths that are not required, or (3) only use the receiver 10 at night. The third option is clearly not acceptable for a robust communication service application. The second option is impractical in some situations due to the high cost associated with large area wavelength selective filters. Also, in the case of the communication wavelength being invisible (in the near infrared for example), the external prime-focus light can be invisible but still contain significant energy.
The baffle approach can also be impractical. If light that enters a telescope with aperture diameter of D at angles of or greater is to be rejected by the baffle, the baffle needs to be placed a distance D/tan( ) in front of the entrance aperture. For example, if D is 0.4 meters and is 5 degrees, the baffle needs to be 4.6 meters in front of the aperture. If this is accomplished by a baffle tube, the system becomes very large and unwieldy.
A baffle containing a plurality of small apertures is mounted on the receiver aperture of an optical receiver, such as a wireless optical telecommunication system receiver. The baffle attenuates ambient radiation that may interfere with the optical receiver. The baffle includes a plurality of apertures that have their axes substantially parallel to the optical axis of the receiver. The depth of the baffle assembly is smaller than a single baffle by the ratio of the small aperture diameter to the full receiver aperture diameter. Honeycomb core material is a practical embodiment of such a multi-aperture baffle.